Alternating hemiplegia of childhood (AHC) is a neurological disorder that usually affects children before 18 months of age. Classic AHC causes recurrent episodes of paralysis (hemiplegia) that involve one or both sides of the body, multiple limbs, or a single limb. The paralysis may affect different parts of the body at different times and may be brief or last for several days. A characteristic feature of AHC is that symptoms disappear during sleep and return upon waking. Many affected children display some degree of developmental delay, abnormal eye (oculomotor) movements, uncontrolled limb movements (including ataxia, dystonia, and choreoathetosis) and seizures. The majority of cases of AHC are caused by a new change (called a pathogenic variant) in the ATP1A3 gene that is not inherited. Thus, most patients with AHC do not have a family history of the disorder. A small number of cases of AHC are caused by changes in the ATP1A2 gene. When this condition does run in families, it follows an autosomal dominant pattern of inheritance.